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Ring non-return valve
Ring non-return valve
The ring non-return valve (NRV) is a screw component that is only available in PSI. In injection moulding screws, it prevents the molten material from flowing back into the screw during the injection process. The NRV has a three-part design. It consists of the pressure ring, the locking tip and the locking ring.
Function
The function of the NRV is basically as follows: During dosing, the material in the screw is conveyed forwards by the hopper and melted in the process. The material flowing forwards presses the laterally movable locking ring against the locking tip. The material flows between the wings of the barrier tip into the screw vestibule. After dosing, the material is pressed down. No further material is conveyed through the screw. The material flows back into the screw due to the pressure gradient. The locking ring is pushed back and hits the cone of the pressure ring. As this is not permeable in contrast to the blade tip, the locking ring and pressure ring close the screw and prevent further backflow.
When dosing starts again, the material conveyed forwards by the auger pushes the locking ring forwards again so that the auger chamber can be filled.
Structure
As mentioned above, the ring non-return valve is basically made up of three parts. In addition to the main components pressure ring, locking ring and locking tip, the centre piece between the pressure ring and locking tip must also be defined in PSI.
The NRV begins with the design of the pressure ring. Its task is to seal the screw against backflowing material when it is pressed down in combination with the locking ring. For this purpose, it has a conical contact surface against which the locking ring presses.
The design of this cone is of decisive importance for the functionality of the lock, as it forms the sealing surface for the locking ring. It is therefore possible to design it in detail in REX.
The centre pieces follow the pressure ring. Depending on the original geometry used, it is possible to assemble the centre piece from different individual geometries.
The centre piece forms the connection between the pressure ring and the barrier tip. Together with the locking ring, the centre piece forms the flow channel for the melt and thus creates a decisive flow cross-section. If it is too large, the pressure on the locking ring is reduced so that it does not slide quickly enough into the locking position. If the flow cross-section is too small, the melt is severely impeded from flowing into the screw vestibule.
The centre section is then followed by the barrier tip. It is designed as a wing tip. This allows the melt to flow through the tip during dosing when the locking ring is in the front position and into the screw vestibule. It should be designed to allow the melt to flow as freely as possible into the screw vestibule, avoiding dead zones. The rear side of the blades serves as a contact surface for the locking ring during dosing. They must be designed accordingly as a contact surface. This applies in particular to the blade angle $β_{Fl}$. This must correspond to the angle of the corresponding contact surface of the locking ring.
The number of vanes is freely selectable. It should be selected in combination with the vane geometry so that the resulting flow cross-section is sufficient for the required amount of melt. As can be seen in the schematic diagram, the geometry is differentiated according to whether a $R_{Flg}$ is specified or not.
The last component is the locking ring. It is movably mounted to open the screw during dosing and to close the screw against backflowing material during injection.
It initially rests against the locking tip or, when closing, against the thrust ring. The contact surfaces of the locking ring are therefore of decisive importance for the function of the NRV. It is important to ensure that the angles of the contact surfaces on both sides are selected so that the surfaces lie flat on top of each other. The outer diameter should be slightly smaller than the inner diameter of the cylinder: On the one hand, the lateral mobility of the ring must be guaranteed. On the other hand, the ring also performs a rotational movement: If it is pressed against the rotating barrier tip during the dosing process, this takes the ring with it and forces it into a rotating movement.
